1. Field of the Invention
This invention concerns an amplifier circuit which is suitable for use as a sense amplifier in a semiconductor memory device.
2. Description of Related Art
Conventionally, amplifier circuits are used as sense amplifiers in semiconductor memory devices. The number of sense amplifiers used increases in proportion to increased integration, especially in DRAM device. Thus, a sense amplifier with low power consumption and which is composed of limited circuit elements is required. To reduce the power consumption and the number of circuit elements used, the inventors attempted to modify the amplifier circuit shown in FIG. 1.
In FIG. 1, P1 and P2 represent P-type MOS transistors and N1 to N3 represent N-type MOS transistors. In this sense amplifier, output signals OUT and OUT are obtained by amplifying the difference in voltage between the bit lines BL and BL when the MOS transistor N3 is activated in response to an enable signal .PHI.. Since the output signals OUT and OUT are achieved by the switching operation of the inverters composed of CMOS circuits, a reduction in the power consumption is achieved. However, in this amplifier circuit, the sensing margin is insufficient, as will be explained in detail below.
FIG. 2 shows a characteristic curve of the amplifier circuit shown in FIG. 1 where the VDD is 5 volts and VSS is ground level. In FIG. 2, the X-axis represents the input voltage and the Y-axis represents the output voltage. BL and BL represent input signals to the amplifier circuit.
The output signals OUT and OUT are achieved as the inverted signals of the input signals BL and BL, when the voltage of the respective input signals exceed the predetermined threshold voltages of the inverter circuits. If the sensing area is defined by an area wherein the amplification factor is more than 3, the sensing area of the circuit shown in FIG. 1 is illustrated by A. Namely, the sensing area is narrow. Thus, the circuit shown in FIG. 1 is insufficient as a sense amplifier in a memory device, the sensing margin being too small with respect to the fluctuation of the input signals.